In this glorious summer of 1983, the Kattegat is an idyll of sailboats. Blue and red spinnakers billow out on the deep green water. An occasional container ship from Archangelsk steams by on its way to the Kiel Canal, and on an upper deck, a Soviet sailor puts on a bravado display of flips on an athletic bar for the benefit of a chance yacht audience.
In island harbors the stray catamaran, Dutch one-and-a-half master, or lovingly handcrafted Viking boat anchors in solitary splendor. All the standard-sized boats cluster at the crowded piers, with Sweds next to Danes next to West Germans.
A discreet etiquette has evolved, in which boaters tread lightly on neighboring decks without disrupting the neighbor's activities.
Except now and then, when it's Swedish and Danish neighbors, and the subject of Hesselo comes up. The waters east of Hesselo, it seems, provide the one lingering dispute from the centuries-long feud between these two Viking descendants.
It's hardly the stuff of which battles of Brunkeberg (1471) or Scania (1709) are made. But Swedish Premier Olof Palme has publicly called the Danes ''autocratic'' and accused them of offending ''vital Swedish interests.'' Danish newspapers are heatedly charging the Swedes with wanting to take away ''our sea'' and ''our oil.''
And at least one Danish yacht owner this summer asserted to one Swedish yacht owner, only partly in jest, that if the latter came from Lund (which has belonged to Sweden since Denmark ceded Scania in 1658), then he must be Danish.
So far, the dispute over the Kattegat line of demarcation shows no sign of escalating beyond the jibes of pleasure sailors and the desultory negotiations that last broke off five years ago. But the two Scandinavian countries are serious in the competing claims to this summer paradise, especially with the whiff of oil in the air.
It was the Danish Energy Ministry's commissioning of oil exploration in the disputed waters this month by the Danish firm of A. P. Moller that first brought the issue to a head.
In an official protest note, the Swedes have asked the Danes to withdraw their several-days-old drilling platform from the disputed waters east of Hesselo. Danish officials have replied that they have no intention of doing so.
Premier Palme and Danish Premier Poul Schluter were scheduled to discuss the impasse at the Scandinavian summit at the fringes of the light athletics world championship today, and the Swedes are talking of taking the dispute to the International Court of Justice in The Hague. Norwegian Foreign Minister Svenn Stray has also offered to mediate if asked.
The Swedish-Danish difference stems from the Law of the Sea Treaty's establishment of the median line between permanently inhabited islands as the proper boundary line for national economic exploitation. Laeso, with its almost 3,000 Danes on 101 square kilometers (around 39 square miles), and Anholt, with its almost 200 Danes on 22 square km (8.5 square miles), clearly qualify, and are so acknowledged by Sweden.
The 0.7 square kilometer of Hesselo, however, with its two caretakers of a business firm's summer cottage, Mr. and Mrs. Sven Jensen, do not qualify in Swedish eyes.
A line drawn between the two countries' mainlands of Scania and Jutland - the alternative to calculating from Hesselo - would fall some 30 km (19 miles) west of the island median line and would give Sweden a considerably larger area of seabed for its own economic exploitation.
Other disputes about comparably inhabited Scandinavian islands include the sea boundaries between the Swedish island of Gotland and the Soviet Union in the Baltic Sea; between Norway's Jan Mayen Island and Denmark's ex-colony of Greenland in the Greenland Sea; and between Norway's Svalbard and the Soviet Union's Novaya Zemlya in the Barents Sea.
In the last case, the Soviet Union has recently broken the understanding of a fishing treaty with Norway covering the disputed waters and set up a drilling platform in these waters to prospect for oil.